The traditional EPROM/Flash EPROM memory cell layout, as shown in a top view in FIG. 1, uses a common source line 12 defined by a diffusion process. Polysilicon ("poly") word lines 9 and 11, usually formed by a photolithography process, run parallel to this source line 12. Field oxide regions 13 and 17, usually formed between nitride regions 18 using a photolithography process, serve to isolate the active regions of the device.
Due to limitations in the oxidation process and the photolithography process used to form field oxide regions 13 and 17, the regions do not exactly match the size and dimensions of a mask 5 used to create them. Instead what occurs is a lateral growing of the field oxide regions 13 and 17 beyond the boundaries of the mask 5, as indicated by reference number 21. This lateral growth/formation is commonly referred to as "bird's beak encroachment" due to its shape.
FIG. 2 shows a side view of the bird's beak encroachment 21. The bird's beak encroachment 21 is created when the field oxide regions 13 and 17 are formed by diffusion into the pad oxide 16. This diffusion process causes the field oxide to also diffuse into the underlying silicon substrate 29 as well as underneath, thus physically lifting, the protective nitride layer 18 as is indicated by reference number 6. It is this diffusion into the silicon substrate 29 and lifting of the nitride layer 18 which creates the field oxide formation that resembles a bird's beak.
A problem with bird's beak encroachment is that it requires the diffusion line width 2, referring again to FIG. 1, to be drawn wider. That is, bird's beak encroachment 21 and 23 of the field oxide regions 17 and 19 requires placing the poly word lines 9 and 11 further apart so as to prevent the encroachment of neighboring field oxide regions 17 and 19 from contacting each other. This separation is necessary in order to leave room for the source line 12. This additional spacing of the diffusion line width 2 can't follow the same scaling factor as technologies advance toward finer geometry because it will consume a larger and larger percentage of the cell real estate as cell sizes become smaller and smaller.
FIG. 3 shows a top view of field oxide region 17. Not only does the field oxide diffusion create a bird's beak 21 which spreads beyond the original mask line 5, but the field oxide region 17 also does not yield square corners. This is initially due to optical resolution limitations in the photolithography process which cause the corners of the photoresist mask pattern to become slightly rounded when the mask shape is transferred to the photoresist. Then, due to the lower amount of oxygen available at a corner than at a side of the field oxide region, lateral diffusion or oxidation occurs more slowly in the corners than in areas relatively distanced from a corner. Photolithography optical resolution limitations and slower lateral oxidation at the corner thus create a corner rounding effect 3 of the bird's beak encroachment 21 of the field oxide region 17.
These corner rounding effects can seriously affect the behavior of an EPROM or Flash EPROM memory device. Referring now to FIG. 4, and with respect to Cell 10, as the edge of poly word line 9 is formed more closely to the ends of field oxide regions 13 and 17 thus reducing distance 1, corner rounding 3 can contribute to an increased gate-to-source tunnelling area, as shown by the enlarged gray area 4. This increased gate-to-source tunnelling area 4 increases the associated tunnel area capacitance which degrades the tunnel erase process. This occurs whenever the polysilicon word lines are misaligned relative to the field oxide regions having rounded corners.
Normally, this mis-alignment poses no serious problem to the fabrication of reliable integrated circuits because it is a relatively simple matter to form the poly word line 9 a large enough distance 1 from the ends of the field oxide regions 13 and 17 to avoid the corner rounding. However, the dead spaces caused by indenting the overlap of the poly word line on the field oxide to avoid corner rounding effects will consume a larger and larger percentage of the cell real estate as cell sizes become smaller and smaller.
As will be seen, the present invention provides a method for forming a source region which is self-aligned with the poly word line as well as an apparatus formed thereby. In the present invention, the end edges of the field oxide regions are vertically aligned with the poly word line with no bird's beak encroachment and corner rounding effect remaining in what will become the source region. The source region, formed between the ends of the field oxide regions of neighboring cells, is thus self-aligned with both the field oxide regions and the poly gate word lines. This self-alignment of the source region allows closer placement of poly word lines without any decrease in source width which thus requires less physical separation between (allows closer placement of) one memory cell and the next memory cell. Reduced cell size and greater overall device density is thus achieved.